1. Field of the Invention
This invention relates to a pneumatic tire for motorcycles possessing good high-speed running stability, gripping property, ground contacting property and rigid feeling.
2. Related Art Statement
As this type of the tire, there are pneumatic radial tires for motorcycles comprising a carcass ply comprised of organic fiber cords arranged at a cord angle of 75.degree.-90.degree. with respect to an equator of the tire and a belt superimposed about an outer periphery of the carcass ply and comprised of a belt layer extending in the widthwise direction of the tire. The belt layer is formed by spirally winding a cord having a modulus of elasticity of not less than 600 kg/mm.sup.2, such as high modulus polyester fiber cord, polyvinyl alcohol fiber cord, rayon fiber cord, aromatic polyamide fiber cord or the like in the circumferential direction of the tire. In these tires, the bulging of the tread outward in the radial direction of the tire through centrifugal force during the high-speed running can be prevented by the hoop effect of the above belt formed by the spiral winding of the cord, so that the high-speed running stability is excellent. Furthermore, since the cords for the belt are aligned in the widthwise direction of the tire by spiral winding, the bending rigidity of the tread in the radial direction of the tire is small, and consequently the ground contacting property becomes excellent.
The radial tire provided with the belt of spirally wound cord layer has many merits as mentioned above, however, the bending rigidity in the radial direction of the tire is small owing to the belt structure itself, so that the laterally rigid feeling is lacking.
In order to enhance the lateral rigidity in the motorcycle tire, it is generally attempted to increase the number of plies constituting the carcass, or to extend the reinforcing member for the bead portion toward the tread end, or the like. However, in the former case, the rigidity as a whole of the tire including not only tread but also sidewall becomes high and particularly the rigidity of the sidewall becomes relatively high. It is unavoidable to degrade the gripping property and the steering property. In the later case, the rigidity of the sidewall becomes excessively high, so that there is a problem that the gripping property and the steering property are degraded in the cornering.